Many wireless communication protocols provide for transmitters, operating within a communication network, which are capable of transmitting at varying levels of output power. One reason for having varying levels of output power is to accommodate transmitters in wireless devices such as wireless telephones, wireless personal data assistants (PDAs), pagers, two-way radios, and other types of wireless devices, which may be located at a varying distance from a base station. In some instances, the wireless communication protocol requires that the signal being received by the base station is received at a relatively constant or fixed power level.
Examples of such protocols include Code Division Multiple Access (CDMA) and Wideband Code Division Multiple Access (WCDMA). To accommodate this requirement, a mobile transmitter will transmit at one of several power output levels, dependent upon the level at which the signal is being received. Other examples, where the transmitted output power can be varied, include Enhanced Data Rates for Global Systems for Mobile Communications Evolution (EDGE) and Global System for Mobile communications (GSM) which provides for a range of output power control of mobile transmitters between 20 dB and 30 dB, which is controllable in steps of 2 dB, and earlier analog cellular standards, which call for seven 4 dB steps in power output of the radio transmitter. Further, multimode wireless devices are designed to transmit communication signals of different modulation schemes using a single power amplifier. Therefore, the single power amplifier must also be capable of transmitting at the power output levels required for each of the different modulation schemes.
The single power amplifier in the wireless device is typically designed to operate most efficiently at the highest power level rather than at the lowest power level. This is because relatively larger amounts of power are consumed when the power amplifiers are operating at the highest power levels than when power amplifiers are operating at lower power levels. Consequently, designing a power amplifier for high efficiency at high output power levels has generally resulted in power amplifiers that sacrificed power efficiencies at lower power levels. However, the wireless device is generally only required to transmit at its maximum power level when the path losses are the greatest. Correspondingly, the wireless device will typically transmit at lower power levels for a larger percentage of the time, that the wireless device is transmitting. Consequently, typical power amplifiers have less than optimal efficiency most of the time that the wireless device is transmitting at low power levels since they were designed to operate most efficiently at high power levels. As a result, these amplifiers do not consume the lowest possible amount of power.
Power amplifiers required to transmit signals of different modulation schemes are typically optimized for operation when using one modulation scheme, such as non-linear modulation. As a result, when the power amplifier is using another modulation scheme, such as linear modulation, the power amplifier is less efficient. For example, in a wireless device operating with linear modulation, such as in CDMA, linear modulation quality is required over a wide power output range. By contrast, in a wireless device operating with non linear modulation, such as in GSM, a constant amplitude GSM signal is required through out the required output power range in order for the power amplifier to operate at high efficiency levels. As a result, if a power amplifier is optimized for linear modulation, the power amplifier will not provide high levels of efficiency when operating with non linear modulation.
One example of a technique for improving the efficiency of a power amplifier at low power levels is to turn off or removing the bias signal from sections of a final stage amplifier in a multi-stage power amplifier. However, turning off sections or removing the bias signal from sections of the final stage amplifier affects other parameters of the power amplifier, such as the optimum output current and voltage operating region, power amplifier loading, and power amplifier output impedance. As a result, this technique alone does not provide optimal performance for minimizing power consumption at low power levels.
Another technique which has been used to enhance operating efficiencies at lower power output values has included reducing the bias signal supplied to the power amplifier. However, there is a limit to the amount that the bias signal can be reduced. Reducing the bias too much will lead to distortion, and therefore increase the likelihood that spurious signals will enter adjacent channels in the communication system. Further, the distortion may negatively affect the ability of a base station receiver to detect the incoming signal from the wireless device transmitter.
A further technique, which has been used to enhance operating efficiencies at lower power output levels of the RF output signal, is to adjust a load impedance coupled to the output of the power amplifier. However, as the power output levels of the RF output signal moves further away from the original maximum required, a greater impedance change becomes necessary to maintain performance.
Still, a further technique has combined varying a bias signal supplied to the power amplifier with varying a load impedance applied to an output of the amplifier. This allows the distortion effect associated with reducing the bias signal to be at least partially mitigated. However, this technique does not provide optimal efficiency for minimizing power consumption.